3DEngine

Uggi3DGPL C#

Uggi3D is a 3D lib written in C# using DirectX.
It makes 3D game development easier by abstracting the underlying API,
and - I'm not so insane and call this a 3D engine (as others do)
- but it has much engine-like stuff (GUI, material system etc...)

Axiom 3D Engine

Open-source, cross-platform 3D rendering engine for .NET.
The Axiom Engine is a high-performance C# port of the powerful OGRE engine.
Its flexible component-oriented architecture allows easy extension
and provides full support for both DirectX and OpenGL

Pulse EngineC#.C++.Python LGPL

The Pulse Engine is a game engine that has been developed over the course of two years.
Along with the Pacemaker toolset,
it contains everything you need to create a game including 3D rendering, audio, networking,
and python scripting.

The Open Toolkit libraryMIT C#

The Open Toolkit is a cross-platform game development library for .Net/Mono.
OpenTK provides intuitive low and high-level wrappers to OpenGL and OpenAL,
high performance timers, virtual controllers, a Math module and more.